default_route_switching.sh

#!/bin/bash

#*********************************************************************
#       Configuration
#*********************************************************************
DEF_GATEWAY="192.168.1.2"       # default route
BCK_GATEWAY="192.168.1.1"       # backup route
RMT_IP_1="8.8.8.8"              # first remote ip to test
RMT_IP_2="8.8.4.4"              # second remote ip to test
PING_TIMEOUT="15"               # ping timeout in seconds
SLEEP_TIME="30"                 # sleep during this time before looping again
BYPASS_DEFAULT_GW_FILE="/tmp/use_default_gateway"  # if this file exists bypass connectivity tests
LOG_FILE="/var/log/default_route_switching.log"           # file to write events into
#*********************************************************************

# check if it's running as root
if [ `whoami` != "root" ]
then
        echo "Failover script must be run as root!"
        exit 1
fi

# looping forever
while true
do
        # get curently used gateway
        CURRENT_GW=`ip route show | grep default | awk '{ print $3 }'`

        # if current gateway is the default one then check directly if it's working
        if [ "$CURRENT_GW" == "$DEF_GATEWAY" ]
        then
                # test connection
                ping -c 2 -W $PING_TIMEOUT $RMT_IP_1 > /dev/null
                PING_1=$?
                ping -c 2 -W $PING_TIMEOUT $RMT_IP_2 > /dev/null
                PING_2=$?
        else
                # else it's currently the backup gateway used, so add route to check if defaut gateway is working

                # add temporary static routes to remote ip's using default gateway
                ip route add $RMT_IP_1 via $DEF_GATEWAY
                ip route add $RMT_IP_2 via $DEF_GATEWAY

                # test connection
                ping -c 2 -W $PING_TIMEOUT $RMT_IP_1 > /dev/null
                PING_1=$?
                ping -c 2 -W $PING_TIMEOUT $RMT_IP_2 > /dev/null
                PING_2=$?

                # delete temporary static route to remote ip's using default gateway
                ip route del $RMT_IP_1
                ip route del $RMT_IP_2
        fi

        # get curent date_time
        LOG_TIME=`date --rfc-3339=s`

        # if forcing default gateway
        if [ -f $BYPASS_DEFAULT_GW_FILE ]
        then
                # if the default gateway is not already set
                if [ "$CURRENT_GW" != "$DEF_GATEWAY" ]
                then
                        # switching to default gateway
                        ip route del default
                        ip route add default via $DEF_GATEWAY

                        # flushing route cache
                        ip route flush cache

                        echo "$LOG_TIME: forced Gateway to Default with IP $DEF_GATEWAY" >> $LOG_FILE
                fi
        #elif [ "$PING_1" == "1" ] || [ "$PING_2" == "1" ]  # pessimistic test, at least one test failed so switch to backup
        elif [ "$PING_1" == "1" ] && [ "$PING_2" == "1" ]  # optimistic test, the two tests failed then switch to backup
        then
                # if one ping failed then switch to backup gateway

                # if the backup gateway is not already set
                if [ "$CURRENT_GW" == "$DEF_GATEWAY" ]
                then
                        # switch to backup gateway
                        ip route del default
                        ip route add default via $BCK_GATEWAY

                        # flushing routing cache
                        ip route flush cache

                        echo "$LOG_TIME: switched Gateway to Backup with IP $BCK_GATEWAY" >> $LOG_FILE
                fi
        elif [ "$CURRENT_GW" != "$DEF_GATEWAY" ]
        then
                # switching to default gateway
                ip route del default
                ip route add default via $DEF_GATEWAY

                # flushing route cache
                ip route flush cache

                echo "$LOG_TIME: switched Gateway to Default with IP $DEF_GATEWAY" >> $LOG_FILE
        fi

        # wait second before looping again
        sleep $SLEEP_TIME
done

what is this in centos 7
BYPASS_DEFAULT_GW_FILE="/tmp/use_default_gateway" # if this file exists bypass connectivity tests

@shahidaslam01

this file will "pause" this script and enforce the system to use the default gateway (DEF_GATEWAY)
having it in the /tmp folder give the permissions to user to perform this action.